Add missing prefix to configure

Without the prefix set to match the dest directory libtool's finish action kills ld.so.cache and the ability to resolve dynamic library mappings rendiering crew completely broken and requiring a bunch of manual triage.

packages/libtool.rb

@@ -8,7 +8,7 @@ class Libtool < Package
   depends_on 'buildessential'
 
   def self.build
-    system "./configure"
+    system "./configure --prefix=/usr/local"
     system "make"
   end